Field devices for measuring and/or monitoring the fill level of a medium in a container are produced and sold by the assignee under the mark “Liquiphant”. Such a measuring device is usually composed of an oscillatable unit, a driving/receiving unit, and a control/regulating unit. The driving/receiving unit excites the oscillatable unit—most often, an oscillatable fork—to oscillate. The frequency (f) of oscillation depends e.g. on whether the oscillatable unit is oscillating in air or covered with medium. Thus, from the frequency, the degree of covering can be deduced. This can also be derived from the amplitude; usually, however, the frequency is evaluated. In the driving/receiving unit, a piezoelectric element is present, for example; this converts an electrical signal into a mechanical oscillation, which then is transferred via a suitable membrane, or diaphragm, to the oscillatable unit. Conversion of the mechanical oscillation into an electric signal occurs in the same way, in reverse. A feedback electronics, which amplifies and feeds the signal of the oscillatable unit back, and the electronics for evaluation and further processing of the oscillation, are combined in a control/evaluation unit. Such fill level measuring devices are usually applied as limit-level switches. In such case, the oscillatable unit is mounted at a determined position, e.g. inside the container, from which a fill level of the medium results. Measured can be either the subceeding, or falling beneath, of this fill height (protection against running empty, or minimum protection or minimum detection) or the exceeding of this fill height (protection against overfilling, maximum protection or detection). In the case of protection against running empty, the oscillatable unit first oscillates in the medium and then in air or e.g. in a second medium with a lesser density in the case of an interface detection (e.g. oil/water). The oscillation frequency in the case of oscillatable unit immersed in the medium, or in the medium with higher density, is less than in the case of oscillation in air or in the medium with the lesser density. Consequently, from the fact that the oscillation frequency becomes greater, or climbs above a certain threshold, it can be concluded that the oscillatable unit is oscillating freely, thus is no longer covered, or that the oscillatable unit is oscillating in the medium with the lesser density. This means that the medium with the higher density—this holds also for the case, from medium to air—has fallen below the fill level. On the basis of this information, for example, a drain can be closed, or an alarm can be triggered. Similar considerations hold for application as a maximum protection.
A problem concerns the formation of accretions. Some media, e.g. foaming liquids, coat the oscillatable unit and deposit on it. This accretion increases the mass of the oscillatable unit. Connected therewith is a lessening of the oscillation frequency (f), i.e., because of the additional mass, the oscillatable unit oscillates with a lesser frequency (f). If such a field device, whose oscillatable unit is coated with accretion, is used for protection against running empty, then there is the danger, that a “covered” report will be issued, even though the fork is actually oscillating freely, because the oscillation frequency (f) is, due to the accretion, clearly below the frequency evaluated as a measure for the fork being free. Safety is, consequently, no longer unconditionally assured; thus, recognizing the presence of accretion is very important. In the case in which the field device is used for protection against overfilling, recognizing accretion is likewise of interest, since the accretion leads also to a “covered” report, when the oscillatable unit is really oscillating freely. Thus, an accretion prevents reliable functioning of the field device.